1. Field of the Invention
This invention relates to a diffraction optical element and an optical system having the diffraction optical element, and particularly to a diffraction optical element for use in light of a plurality of wavelengths or bands and an optical system using the same.
2. Related Background Art
In a refracting optical system, use has heretofore been made of a method of decreasing chromatic aberration by a combination of glass materials differing in dispersion. In contrast, a method of decreasing chromatic aberration by providing a diffractive optical element (hereinafter referred to also as DOE) having the diffracting action on a lens surface or in a portion of an optical system is disclosed in such literature as SPIE, Vol. 1354, International Lens Design Conference (1990), Japanese Patent Application Laid-Open No. 4-213421 (corresponding U.S. Pat. No. 5,044,706), Japanese Patent Application Laid-Open No. 6-324262, etc. These utilize the physical phenomenon that on the refracting part and the diffracting part of the optical system, chromatic aberration appears in opposite directions.
Further, such DOE can also be given an effect like that of an aspherical lens by varying the period of the periodic structure thereof, and is greatly effective to reduce aberrations.
On the other hand, a ray refracted by a refracting surface is still a ray after being refracted, whereas in the DOE, a ray has the nature that the ray is divided into lights of respective orders after being refracted. Therefore, when the DOE is used in a lens system, such grating design that a beam of a wavelength area used concentrates in a particular order (hereinafter referred to also as the design order) is indenspensable. Specifically, it is necessary that, in the entire area of the wavelength used in the system, the diffraction efficiency of a ray of the design order be sufficiently high.
When there is present a ray having other diffraction order than the design order, it becomes a flare light which adversely affects image performance at a location whereat light of the original design order is imaged. Accordingly, what is important in the construction of an optical system using the DOE is to sufficiently take into consideration the spectral distribution of the diffraction efficiency at the design order and the behavior of rays of other orders than the design order.
The characteristic of the diffraction efficiency of a DOE as shown in FIG. 10 of the accompanying drawings for diffracted light of the particular diffraction order is shown in FIG. 11 of the accompanying drawings. In FIG. 10, the DOE 1 is such that on a substrate (base) 2, a phase-type diffraction grating 4 formed of a predetermined material is formed into a sawtooth cross-sectional shape with a grating height (depth) d.
The value of the diffraction efficiency is the rate of the quantity of each diffracted light to the entire transmitted beam, and is a value not taking the reflection or the like on the boundary surface of the grating into account because description will become complicated if it is taken into account. In FIG. 11, the axis of abscissas represents wavelength and the axis of ordinates represents diffraction efficiency. This DOE is designed such that in the first diffraction order (solid line in FIG. 11), diffraction efficiency becomes highest in the wavelength area used (that is, the design order is the first order).
Further, the diffraction efficiency for the diffraction orders in the vicinity of the design order (zero order and second order with respect to the + first order (design order)) is also shown. As shown in FIG. 11, at the design order, the diffraction efficiency becomes highest for a certain wavelength (hereinafter referred to as the design wavelength) and gradually becomes lower for the other wavelengths. The amount of reduction in the diffraction efficiency of the design order for the other wavelengths than the design wavelength becomes diffracted lights of the other orders and becomes a factor of flare. Also, when a plurality of DOE""s are used, the reduction in the diffraction efficiency also leads to a reduction in transmittance.
Various propositions have been made as constructions for restraining the reduction in the diffraction efficiency for the other wavelengths than the design wavelength which becomes such a factor of flare. In a construction disclosed in Japanese Patent Application Laid-Open No. 9-127322 (corresponding U.S. Pat. No. 6,157,488), as shown in FIG. 12 of the accompanying drawings, three different materials and two different grating thicknesses (d1 and d2) are optimally chosen and are disposed in proximity to each other at equal pitch distributions to thereby realize high diffraction efficiency in the entire visible area of the design order, as shown in FIG. 13 of the accompanying drawings.
Also, Japanese Patent Application Laid-Open No. 10-133149 discloses a DOE having a laminated cross-sectional shape in which diffraction gratings are superposed in two layers, as shown in FIG. 14 of the accompanying drawings. The refractive indices, dispersion characteristics and grating thicknesses of materials forming the diffraction gratings of the two layers are optimized to thereby realize high diffraction efficiency in the entire visible area of the design order.
Also, Japanese Patent Application Laid-Open No. 8-220482 presents a construction in a relief-type DOE having a sawtooth cross-sectional shape in which flare caused by the wavelength dependency of diffraction efficiency is improved. That is, the relief pattern surface of the DOE is divided into areas and the optimization of the depth of grooves in the relief pattern surface of the pertinent area is effected so that diffraction efficiency may become maximum for the central wavelength of light passing through the pertinent area. In addition, as shown in FIG. 15 of the accompanying drawings, diffraction efficiency is improved in the vicinity of the design wavelengths xcexa, xcexb and xcexc to thereby reduce flare. Also, in Japanese Patent Application Laid-Open No. 10-104411 (corresponding to U.S. Pat. No. 6,011,651), the design wavelength of a kinoform-type diffractive optical element as shown in FIG. 10 of the accompanying drawings is set to a suitable value at which it is difficult for color flare to be conspicuous, and specifically the grating thickness is adjusted, thereby reducing the quantity of unnecessary diffracted lights in the vicinity of the design wavelength.
However, in the above-mentioned Japanese Patent Application Laid-Open No. 9-127322 and Japanese Patent Application Laid-Open No. 10-133149, it is only described that the diffraction efficiency for the design order is greatly improved and therefore unnecessary diffracted lights of the other orders than the design order are greatly reduced and flare is decreased, and there is no detailed description of the color taste of flare and the amount of flare.
On the other hand, Japanese Patent Application Laid-Open No. 8-220482 shows a construction in which in a DOE of single layer construction (hereinafter referred to as the single layer DOE) provided by a relief-type diffraction grating having a sawtooth-like cross-sectional shape, the central wavelength of transmitted light through each area is the design wavelength, whereby the wavelength dependency of diffraction efficiency is reduced, but there is made no description of a DOE having a laminated cross-sectional shape (hereinafter referred to as the laminated DOE) in which diffraction gratings are superposed in two or more layers.
Also, Japanese Patent Application Laid-Open No. 10-104411 bears a description of the influence, etc. of the color flare of lights of unnecessary orders, but the DOE used in the description is a single layer DOE as shown in FIG. 10, and no mention is made of the flare regarding the single layer DOE.
In an optical system using the aforedescribed laminated DOE, in contrast with the single layer DOE, flare is greatly reduced, but it cannot be said that unnecessary diffracted lights are not at all present, and slightly, unnecessary diffracted lights remain. In the application of the laminated DOE to an optical system in which the photographing (projecting) condition does not change (for example, the reader lens of a copier or the projection lens of a liquid crystal projector), the influence of flare can be restrained to a problem-free level by the laminated DOE. In contrast, in an optical system for photographing an object under various conditions such as a camera or a video camera, it has become apparent that the slightly remaining flare poses a problem. Particularly when the flare component has wavelength dependency, color flare similar to that in the single layer DOE of Japanese Patent Application Laid-Open No. 10-104411 occurs. On the other hand, in the laminated DOE of Japanese Patent Application Laid-Open No. 11-64616 (corresponding to U.S. Pat. No. 6,122,104), there are three or more design wavelengths which are wavelengths (peak wavelengths) for which diffraction efficiency becomes maximum. According to this DOE, the occurrence of color flare is considerably restrained.
So, the present invention has as its object to provide a diffractive optical element improved over the element of Japanese Patent Application Laid-Open No. 11-64616 and an optical system having such diffractive optical element.
In order to achieve the above object, in the diffractive optical element of the present invention, three layers each having a relief-type grating are formed of different materials, and at least three diffraction optical parts are provided in the boundary areas of the respective layers. The diffractive optical element is characterized in that the material forming each layer and the grating height are appropriately set so that in three wavelengths which are the main wavelengths of the three primary colors, the diffraction efficiency for diffracted light of a predetermined order may be maximum.
Here, when the diffractive optical element of the present invention is used in an optical system for forming an image on the photosensitive surface of silver salt film, an image pickup element or the like, the main wavelengths of the three primary colors refer to the main wavelengths of color lights for which the spectral sensitivity of the photosensitive surface is high. Also, when the diffractive optical element of the present invention is used in an optical system for illuminating an original picture with light from a light source, and projecting the image of the illuminated original picture, the main wavelengths of the three primary colors refer to the main wavelengths of color lights included in the light from the light source.